The incorporation of whisker or fiber materials in a ceramic, glass or metal matrix has often resulted in improved mechanical properties of the resulting composites. The use of reinforcing whiskers to improve the fracture toughness of structural ceramics such as silicon carbide or silicon nitride has been advocated so as to make these materials more resistant to the mechanical and thermal stresses inherent in exacting applications such as advanced vehicular engine components. In making ceramic composites for these demanding applications it is preferred to use whiskers of the same composition so as not to compromise the excellent material properties of a monolithic ceramic such as silicon nitride by the admixture of a material with inferior characteristics. However, silicon nitride whiskers of the requisite purity for ceramic engine applications are not available.
Thus, a commercially available silicon nitride whisker product made by Tateho Chemical Industries of Japan contains large amounts of impurities, particularly calcium and iron, which adversely affect the high-temperature strength and oxidation resistance of silicon nitride. Another process for the growth of Si.sub.3 N.sub.4 whiskers (J. V. Milewski, F. D. Gac and J. J. Petrovic, Rpt. DE83008282, Los Alamos National Laboratory, Feb. 83) uses stainless steel as a catalyst which also results in unacceptable levels of iron contamination. Other techniques alluded to in an article by W. P. Clancy [Microscopy, 22, 279, (1974)] consist either of high temperature and high pressure (1800.degree. C. at 275 psi) or long treatment (24 to 48 hours) at 1425.degree. C., neither of which is convenient for industrial production. Furthermore, the purity of the latter materials was not reported.